A current sensor (or “current detecting apparatus”) disclosed in Patent Document 1 indicated below is known as a current sensor of this type. This current sensor is equipped with a ring-shaped coil, a printed circuit board on which a chip-type noise filter and chip resistors for dividing a voltage are mounted, a non-magnetic shield case which is made of aluminum or the like and internally houses the ring-shaped coil and the printed circuit board, and an external case that is non-magnetic and insulating and is made of resin or the like.
The printed circuit board has an input-side conductive pattern, a relaying conductive pattern, an output-side conductive pattern, and an earthed conductive pattern on one surface. The chip-type noise filter is a “T filter” composed of two inductors and one capacitor, the two ends of the series circuit composed of the inductors are respectively connected to the input-side conductive pattern and the relaying conductive pattern, and the two ends of the capacitor are respectively connected to the connection point between the inductors and the earthed conductive pattern. A 47Ω chip resistor (i.e., one resistor out of the voltage-dividing resistors) is mounted on the printed circuit board so as to connect the relaying conductive pattern and the output-side conductive pattern, and three chip resistors (i.e., the remaining resistors out of the voltage-dividing resistors) are mounted on the printed circuit board so as to connect the relaying conductive pattern and the earthed conductive pattern. These three chip resistors each have a resistance of 12Ω, and by connecting these three resistors in parallel, an overall resistance of 4Ω is produced.
A lead that extends from a detection winding of the ring-shaped coil is connected to the input-side conductive pattern and the other lead is connected to the earthed conductive pattern. The output-side conductive pattern is connected via a lead to the core wire at a first end of a coaxial cable and the earthed conductive pattern is connected to the earth wire at the first end of the coaxial cable.
With this configuration, by passing a wire to be detected through the ring-shaped coil of the current sensor so that the wire is used as the primary winding of a current transformer, a current that is proportional to the current on the detected wire will be induced in the detection winding of the ring-shaped coil that functions as the secondary winding of the transformer. After removal of high-frequency noise by the chip-type noise filter, the induced current is converted to a voltage by the three chip resistors that are connected in parallel so as to construct a 4Ω resistor. The converted voltage is also outputted to the core wire of the coaxial cable via the 47Ω chip resistor. With this configuration, the three chip resistors that construct a 4Ω resistor and the 47Ω chip resistor together function as an approximately 50Ω resistor, and it is believed that matching the impedance when looking from the input end of the coaxial cable used as a transmission path toward the noise filter to the input impedance of the coaxial cable (i.e., the characteristic impedance (50Ω) of the transmission path) will reduce disturbances occurring in the waveform of the signals transmitted through the coaxial cable.